1. Field of the Invention
The present invention relates to a display device including a reflector and more particularly relates to a reflective color display device that ensures increased lightness for color white displayed.
2. Description of the Related Art
Recently, reflective color display devices have rapidly expanded their applications and now found them in various types of mobile electronic units including cell phones, portable game appliances and so on. A reflective color display device has a number of advantages over a transmissive color display device. Specifically, since a reflective color display device needs no backlight, the light source power can be cut down and the space and weight of the backlight required for a transmissive color display device can be saved. In addition, the overall power dissipation of a reflective display device can also be much lower than that of a transmissive display device, thus allowing the user to carry a downsized battery. For these reasons, the reflective color display device is not only effectively applicable to various types of mobile electronic units that should be as light and as thin as possible but also allows the use of a battery of an increased size when a unit including the reflective display device is designed to have the same size and weight as a conventional one. Thus, the reflective display device is expected to increase the longest operating time of those units by leaps and bounds.
A reflective color display device like this also ensures good contrast on its display. When a CRT, i.e., a self-light-emitting display device, or a transmissive color liquid crystal display device is used outdoors under the sun, the contrast ratio thereof decreases considerably. On the other hand, a reflective color display device increases the lightness of the image displayed thereon proportionally to the quantity of ambient light, thus realizing a good contrast ratio. For that reason, a reflective color display device is particularly suitable to outdoor use.
Hereinafter, a configuration for a conventional reflective color liquid crystal display device will be described.
In a reflective liquid crystal display device used extensively today, one or two polarizers are used and color filters are arranged side by side. A reflective liquid crystal display device like this may operate in one of the following three modes:
Twisted nematic (TN) mode in which a display operation is conducted by controlling the optical rotatory power of the liquid crystal layer;
Electrically controlled birefringence (ECB) mode in which a display operation is conducted by controlling the birefringence of the liquid crystal layer by an electric field; and
A mixed mode as a combination of the TN and ECB modes.
A conventional reflective display device cannot achieve sufficiently high display quality (in the respect of brightness, in particular). This is because the optical efficiency of the polarizers and laterally arranged color filters is as low as 50% or less. As a result, the conventional reflective device cannot ensure a reflectance high enough to realize a bright image as required.
Thus, to increase the reflectance, reflective display devices requiring no polarizers or color filters have been proposed.
Examples of reflective display devices with no polarizers include a liquid crystal display device (LCD) using a guest host liquid crystal material to which a dye has been added, an LCD using a polymer-dispersed liquid crystal material, and an LCD using a cholesteric liquid crystal material. On the other hand, a color display device, in which three display panels for three different colors are stacked one upon the other, has been developed as a type including no horizontally arranged color filters. Display devices of this type are disclosed in Society of Information Display ""98 Digest (p. 897) and Japanese Laid-Open Publication No. 10-260427, for example.
However, the process of fabricating a single display device by stacking three panels one upon the other is overly complicated. What is worse, the incoming light is differently modulated by the respective liquid crystal layers in those stacked panels, thus increasing the resultant parallax easily. To reduce this parallax, the thickness of an intermediate substrate, located between adjacent panels, should be sufficiently smaller than the size of each pixel. Accordingly, the intermediate substrates need to be made of a film-like material. Nevertheless, it is very difficult to form active elements like TFTs on the film-like material.
The display device disclosed in Society of Information Display ""98 Digest (p. 897) uses a liquid crystal material having memory properties to conduct a display operation by a passive-matrix addressing technique requiring no active elements. Thus, the intermediate substrates may be relatively thin. However, even in the passive-matrix addressing method, a transparent electrode should also be formed on each of the intermediate substrates. Accordingly, the thickness of the transparent substrates can be no smaller than about 100 xcexcm, which is approximately equal to the size of each pixel. Consequently, the parallax problem is still insoluble by this technique, either.
The display device disclosed in Japanese Laid-Open Publication No. 10-260427 adopts an active-matrix addressing technique. In this display device, TFTs (i.e., exemplary active elements) are arranged on the backmost substrate. In addition, this display device uses a connector electrode that extends through all of those panels stacked, thereby realizing an active-matrix addressing on the stacked panels. However, the transparent electrode should also be formed on each of the intermediate substrates. Accordingly, the thickness of each intermediate substrate cannot be much smaller than the size of each pixel, thus also causing a parallax disadvantageously.
On the other hand, a reflective color display device including a corner reflector is disclosed in International Publication Number WO 98/57212. In the corner reflector, the incoming light undergoes total internal reflection at the three facets thereof. In this case, the incoming light is modulated by getting the reflectance of each of these facets controlled independently. In conducting a color display operation using this reflective display device, there is no need to stack the three types of panels, thus eliminating the parallax problem. In this display device, however, the reflectance of each facet of the corner reflector is controlled by moving a member located behind the facet. More specifically, the reflectance of each facet is controllable independently by moving the member between a position at which the member is in contact with the facet and a position at which the member is separated from the facet by a distance approximately equal to the wavelength of visible radiation. It is difficult for a device of this type to realize a high-definition display operation.
In order to overcome the problems described above, the present invention provides a display device for conducting a high-definition display operation by ensuring sufficiently high contrast ratio and brightness and by eliminating the parallax problem even though the device is reflective.
A display device according to an aspect of the present invention includes a reflector in which a plurality of elements, each including a number of reflective regions, is arranged. In the reflector, the reflective regions of each said element are disposed so that at least part of incoming light, which has been incident on the element, is reflected by each one of the reflective regions after another and then allowed to go out of the element. At least one of the reflective regions of each said element includes: a light reflective plane; and a light modulating layer that is formed on one side of the light reflective plane so as to face the incoming light.
In a preferred embodiment of the present invention, each of the reflective regions of each said element includes the light reflective plane and the light modulating layer, and the light modulating layers included in each said element modulate the incoming light in mutually different wavelength ranges.
In another preferred embodiment, the light reflective plane is a surface of a metal layer.
In still another preferred embodiment, the light reflective plane is a boundary between two types of materials having mutually different refractive indices.
In yet another preferred embodiment, the light reflective plane includes a planar portion.
In this particular embodiment, each said element preferably includes the three planar portions that are opposed perpendicularly to each other to form a corner cube.
More specifically, each of the three planar portions that make up the corner cube preferably has a substantially square shape. In each said element, the three planar portions of the corner cube are opposed mutually adjacently and perpendicularly to each other to define three sides of a single cube that share one vertex.
In that case, the three light modulating layers, disposed on the three sides of each said cube that are opposed perpendicularly to each other to share one vertex thereof with each other, may modulate the incoming light in the same wavelength range.
Alternatively, the reflective regions may be disposed on a single continuous curved surface.
In yet another preferred embodiment, each said light modulating layer may switch from a state of absorbing part of the incoming light failing within a selected wavelength range into a state of transmitting another part of the incoming light falling within a wavelength range that includes at least the selected wavelength range, or vice versa.
A display device according to another aspect of the present invention includes a reflector in which a plurality of elements, each including three reflective regions, is arranged. In the reflector, the three reflective regions of each said element are disposed so that at least part of incoming light, which has been incident on the element, is reflected by each one of the three reflective regions after another and then allowed to go out of the element. Each of the three reflective regions of each said element includes: a light reflective plane; and a light modulating layer that is formed on one side of the light reflective plane so as to face the incoming light.
In a preferred embodiment of the present invention, the display device further includes means for separately driving the three light modulating layers included in the three reflective regions of each said element.
In another preferred embodiment of the present invention, the light modulating layer included in a first one of the three reflective regions of each said element is a host liquid crystal layer including a guest that absorbs red. The light modulating layer included in a second one of the three reflective regions is a host liquid crystal layer including a guest that absorbs green. And the light modulating layer included in the other, third reflective region is a host liquid crystal layer including a guest that absorbs blue.
In an alternative embodiment, the light modulating layer included in a first one of the three reflective regions of each said element may include: a switching layer changing from a state of selectively reflecting red into a state of selectively transmitting red, or vice versa; and a color filter absorbing red. The light modulating layer included in a second one of the three reflective regions may include: a switching layer changing from a state of selectively reflecting green into a state of selectively transmitting green, or vice versa; and a color filter absorbing green. And the light modulating layer included in the other, third reflective region may include: a switching layer changing from a state of selectively reflecting blue into a state of selectively transmitting blue, or vice versa; and a color filter absorbing blue.
In this particular embodiment, each said switching layer may be made of a cholesteric liquid crystal material.
Alternatively, each said switching layer may also be made of a holographic polymer-dispersed liquid crystal material.
A display device according to still another aspect of the present invention includes a reflector in which a plurality of elements, each including a number of reflective regions, is arranged. In the reflector, the reflective regions of each said element are disposed so that at least part of incoming light, which has been incident on the element, is reflected by each one of the reflective regions after another and then allowed to go out of the element. At least one of the reflective regions of each said element includes a light modulating layer that is changeable between at least two states in which light, falling within a particular wavelength range selected from the visible range, is absorbed to mutually different degrees.
In a preferred embodiment of the present invention, the light modulating layer has a thickness approximately equal to or greater than the wavelength of visible radiation.
In another preferred embodiment of the present invention, the light modulating layer changes its states when a voltage is applied thereto.
In this particular embodiment, the display device preferably further includes an electrode for changing the states of the light modulating layer.
More specifically, the light modulating layer preferably contains a substance that absorbs the light falling within the particular wavelength range, and a physical state of the substance preferably changes when the voltage is applied thereto.
Alternatively, the light modulating layer may contain a substance that absorbs the light falling within the particular wavelength range, and a position of the substance may change when the voltage is applied thereto.
In still another preferred embodiment, the light modulating layer may contain a substance that reflects visible radiation.
In yet another preferred embodiment, the light modulating layer may include: a medium; a first type of particles, which are dispersed in the medium, absorb the light falling within the particular wavelength range and are movable in the medium; and a second type of particles, which are also dispersed in the medium and reflect visible radiation. A degree to which the light modulating layer absorbs the light falling within the particular wavelength range may be controlled by the movement of the first type of particles.
In yet another preferred embodiment, the light modulating layer includes a rotator that is changeable from a state of absorbing the light falling within the particular wavelength range into a state of reflecting the visible radiation, or vice versa, when rotates.
In this particular embodiment, the rotator may be a particle including multiple parts that have mutually different optical properties.
A display device according to yet another aspect of the present invention includes a reflector that includes a concave portion reflecting at least part of incoming light a number of times. A light modulating layer, which is changeable between at least two states in which light, falling within a particular wavelength range selected from the visible range, is absorbed to mutually different degrees, has been formed in the concave portion of the reflector.
A display device according to yet another aspect of the present invention includes a reflector that includes a concave portion reflecting at least part of incoming light a number of times. First and second light modulating layers have been formed in the concave portion of the reflector. The first light modulating layer is changeable between at least two states in which light, falling within a first wavelength range selected from the visible range, is absorbed to mutually different degrees. The second light modulating layer is changeable between at least two states in which light, falling within a second wavelength range selected from the visible range, is absorbed to mutually different degrees. The second wavelength range is different from the first wavelength range. A spectral distribution of the incoming light is changeable by the first and second light modulating layers.